Fully electric and hybrid (for example, gas and electric and fuel-cell and electric) vehicles in existence today typically require charging via plug-in cables that are manufactured in accordance with international standards. (The term “electric vehicle” as used here is intended to encompass both fully electric and hybrid vehicles.) This conductive connection requires the vehicle operator to plug a charging cable into the vehicle. The cable must remain connected to the vehicle during the charging process. One disadvantage of this approach is that it requires the use of high voltage cables. Frayed or damaged cables create a hazardous condition and can cause an electric shock. Because the cable is repeatedly inserted and removed from a receptacle, cycle life is an issue. Maintenance of public charging stations is another concern, especially in winter climates where reliability and accessibility could become issues in icy and snowy weather. The cables must be manufactured to be able to withstand any kind of environmental condition. A plug-in system is also inconvenient as the vehicle operator must plug and unplug the charging unit from the vehicle.
Wireless charging systems have been proposed in response to the aforementioned disadvantages and inconveniences. Two kinds of wireless charging are being investigated: inductive charging and magnetic resonance power generation. Inductive charging uses an alternating electromagnetic field generated by the charging coils to send and receive energy. A magnetic coil in a charging base station creates an alternating electromagnetic field and a second induction coil in a portable device having a battery receives power from the electromagnetic field and converts it into electrical current to charge the battery. Inductive charging carries a much lower risk of shock because there are no cables or exposed conductors. The ability to fully enclose the charging connection makes inductive charging attractive where water impermeability is required. For example, low power (i.e., 3 kilowatts or less) inductive charging is used for implanted medical devices and for electric hygiene devices, such as toothbrushes and shavers that are frequently used near water. Inductive charging makes charging electric vehicles more convenient because it eliminates having to connect a power cable. Some disadvantages of inductive charging are its lower efficiency and increased resistive heating in comparison to plug-in systems. Implementations using lower frequencies or older drive technologies charge more slowly and generate heat within most portable electronics. Inductive charging also requires drive electronics and coils, increasing the complexity and cost of manufacturing.
Because there can only be a small gap between the two coils, inductive charging is considered a short-distance wireless charging system. Newer approaches to inductive charging reduce transfer losses by using ultra thin coils, higher frequencies, and optimized drive electronics. These newer technologies provide charging times comparable to wired approaches and have been employed in vehicle charging. Large and small paddle inductive charging systems (Called Magne Charge LPI and SPI respectively) have been used in conjunction with battery powered electric vehicles (BEV) formerly made by General Motors. However, General Motors withdrew support for the system after the California Air Resources Board settled on a different conductive charging interface for electric vehicles in California. The Magne Charge system (also known as J1773) used high-frequency induction to deliver high power (more than 10 kW) at an efficiency of 86% (6.6 kW power delivery from a 7.68 kW power draw). Other inductive charging systems have been proposed that eliminate cables entirely. For example, U.S. Pat. No. 5,703,461 (Monoshima et al.) discloses an inductive charging system in which the secondary, or receiver, coil is mounted at a specified location under the rear of the vehicle and the primary, or transmission, coil is located above ground on an arm that is able to align the coils without interference.
Non-resonant coupled inductive charging systems work on the principle of a primary coil generating a magnetic field and secondary coil subtending as much as possible of that field so that the power passing though the secondary coil is as close as possible to that of the primary. The requirement that the magnetic field generated by the primary coil be covered by the secondary coil results in a very short range. Over greater ranges, the non-resonant induction method is highly inefficient as the majority of the energy is in resistive losses of the primary coil.
Using magnetic resonance power generation helps increase efficiency dramatically. If resonant coupling is used, each coil is capacitively loaded so as to form a tuned LC (the L stands for inductor and the C stands for capacitor) circuit. If the primary and secondary coils are resonant at a common frequency, significant power may be transmitted between the coils at reasonable efficiency over a range of a few times the coil diameters. The general principle is that if a given oscillating amount of energy is placed into a primary coil that is capacitively loaded, the coil will ‘ring’, and form an oscillating magnetic field. The energy will transfer back and forth between the magnetic field in the inductor and the electric field across the capacitor at the resonant frequency. This oscillation will die away at a rate determined by the Q Factor, mainly due to resistive and radiative losses. However, provided, the secondary coil absorbs more energy than is lost in each cycle of the primary, then most of the energy can still be transferred. The primary coil forms a series RCL circuit (the R stands for resistor), and the Q factor for such a coil is:
  Q  =            1      R        ⁢                            L          C                    .      So, if R=10 ohm, C=1 micro farad, and L=10 mH, the Q Factor is 1000. Because the Q factor can be very high, (experimentally around 1000 has been demonstrated with air cored coils, see Kurs, et al., Wireless Power Transfer via Strongly Coupled Magnetic Resonances, Science 317: 83-86 (2007) and United States Patent Publication No. 2010/010909445 entitled Wireless Energy Transfer Systems) only a small percentage of the field has to be coupled from one coil to the other to achieve high efficiency and the primary and secondary can be several diameters apart. Because the Q can be very high even when low power is fed into the transmitter coil, a relatively intense field can build up over multiple cycles, which increases the power that can be received. At resonance, far more power is in the oscillating field than is being fed into the coil, and the receiver coil receives a percentage of that power. The voltage gain of resonantly coupled coils is proportional to the square root of the ratio of secondary and primary inductances. See also, Wireless Power Minimized Interconnection Problems, Power Electronics Technology: 10-14 (July 2011).
In either case, inductive or magnetic resonance power generation, the alignment of the coils is thought to be critical. (But see, Villa, et al., High-Misalignment Tolerant Compensation Topology for ICPT Systems, IEEE Transactions on Industrial Electronics 59: 945-51 (February 2012)) Certain self-aligning methods for inductive power chargers are disclosed in U.S. Pat. No. 5,646,500 (Wilson) and U.S. Pat. No. 5,498,948 (Bruni and Davenport), both of which are assigned to Delco Electronics. The '500 patent discloses a light-activated mechanical positioning system for aligning the primary (i.e., transmission) and secondary (i.e., receiver) coils. A light source is disposed on the electric vehicle and an array of photoelectric detectors is disposed adjacent the charging coil. The detector array is coupled with a servo control system that includes an X-Y-Z mechanical driver that moves the position of the charging coil in the X, Y, or Z direction based upon signals provided by the array. The '948 patent discloses a slidable alignment mechanism composed of a series of coupled, vertical, horizontal and lateral slide mechanisms that are coupled to the primary coil and to a series of aligning plates disposed at predetermined locations around the coil. Rollers disposed adjacent the secondary coil on the vehicle cause the aligning plates to move if the primary (charging) and secondary coils are misaligned. Similarly, United States Patent Publication 2001/0221387 discloses an inductive energy transfer system in which the charging coil is coupled with a plurality of step-up motors and lead screws that mechanically drive the coil in the x, y, and z-dimensions until the charging coil and the secondary coil are properly aligned. A sensor adjacent the charging coil detects the strength of the magnetic field emanating from the secondary coil and aligns the primary coil in the x, y, and z-directions to produce maximum power. Each of these systems attempt to address the proper engagement of primary and secondary coils that are already in close proximity to each other—they are above ground, located near the front of the vehicle, and use some kind of armature to move into a receptacle to create an inductive coupling.
The challenge in implementing any of these technologies as a practical wireless charging solution requires that the primary and secondary charging coils are aligned well enough for acceptable efficiency as compared with a plug-in charging system. Alignment of the coils is further complicated by the fact that no standard exists among manufacturers relating to the position of the secondary coil on the vehicle and no methodology appears to exist to verify and optimize performance, safety and communication systems used in wireless electric vehicle charging systems. The variety of inductive and magnetic resonance methods have been proposed for wireless charging, they all have varying interoperability, performance (such as efficiency of charging), communications, and safety parameters. It would be advantageous to have a charging system that would work with most any vehicle regardless of the mounting location of the secondary coil. What is required is an apparatus for properly aligning the charge coils of electric vehicle in a charging station regardless of the location of the secondary coil. It would be highly advantageous if that apparatus could also be employed to test the charging system and validate and optimize the parts of the system before and after installation.